Intermolecular reactions catalytic systems

As shown in the manganese- and ruthenium-catalyzed intermolecular nitrene insertions, most of these results supposed the transfer of a nitrene group from iminoiodanes of formula PhI=NR to substrates that contain a somewhat activated carbon-hydrogen bond (Scheme 14). Allylic or benzylic C-H bonds, C-H bonds a to oxygen, and very recently, Q spz)-Y bonds of heterocycles have been the preferred reaction sites for the above catalytic systems, whereas very few examples of the tosylamidation of unactivated C-H bonds have been reported to date. 

Following the success with cobalt and rhodium, Shibata reported Ir(i)-based enantioselective catalytic reaction. Right after their observation that the efficiency of [IrCl(COD)]2-catalyzed PKR substantially increased by addition of a phosphane co-ligand, they moved directly to use chiral phosphanes and examined the enantioselectivity. " TON and TOE of the reaction were low and the number of examples was limited. Typically, the reaction required a fair amount of Ir(i) catalyst [IrCl(COD)]2 (0.1-0.15 equiv.) and (reaction time. However, this has remained as the best in terms of enantioselectivity to date. Moreover, this catalytic system provided the first asymmetric intermolecular reaction as well. 

The inclusion of a separate chapter on catalysed cyclopropanation in this latest volume of the series is indicative of the very high level of activity in the area of metal catalysed reactions of diazo compounds. Excellent, reproducible catalytic systems, based mainly on rhodium, copper or palladium, are now readily available for cyclopropanation of a wide variety of alkenes. Both intermolecular and intramolecular reactions have been explored extensively in the synthesis of novel cyclopropanes including natural products. Major advances have been made in both regiocontrol and stereocontrol, the latter leading to the growing use of chiral catalysts for producing enantiopure cyclopropane derivatives. 

A ruthenium based catalytic system was developed by Trost and coworkers and used for the intermolecular Alder-ene reaction of unactivated alkynes and alkenes [30]. In initial attempts to develop an intramolecular version it was found that CpRu(COD)Cl catalyzed 1,6-enyne cycloisomerizations only if the olefins were monosubstituted. They recently discovered that if the cationic ruthenium catalyst CpRu(CH3CN)3+PF6 is used the reaction can tolerate 1,2-di- or tri-substituted alkenes and enables the cycloisomerization of 1,6- and 1,7-enynes [31]. The formation of metallacyclopentene and a /3-hydride elimination mechanism was proposed and the cycloisomerization product was formed in favor of the 1,4-diene. A... 

We have performed several studies on the manganese catalytic system in order to get some insight to the mechanism. We have observed exchange of alcohols during the reactions involving primary alcohols, which we attributed to occur as a result of proton transfer. We have concluded from our results that the proton transfer process could not be taking place by an intermolecular process and therefore we proposed an intramolecular version to account for the proton transfer. 

It should be noted that the development of catalytic systems has been most successful in the intramolecular reaction, with limited progress being made with the intermolecular variant. 

Following their works on immobilized heterobimetallic nanoparticle catalysts, Chung s group has synthesized Ru/Co nanoparticles immobilized in charcoal and shown the ability of this system to catalyze a PKR-type reaction in the presence of pyridylmethyl formiate as a CO source. They used these conditions with intra- and intermolecular reactions and showed that the catalyst can be reused without loss of catalytic activity (Scheme 40) [146]. 

Catalytic Processes. Catalytic processes lead to intramolecular and intermolecular C-C bond constructions which are usually directly analogous to the stoichiometric reactions. This topic was reviewed in 1983. Catalytic processes often lead to reduction rather than alkene regeneration this is more likely to happen with B12 as a catalyst than it is with a cohaloxime. Schef-fold pioneered the use of vitamin B12 as a catalyst for C-C bond formation, and Tada pioneered the use of model complexes such as cobaloximes. Several of the reactions described in the section on stoichiometric reactions have also been performed cat-aly tically, as mentioned in that section. Commonly used chemical reductants include Sodium Bomhydride and Zinc metal. Electrochemical reduction has also been used. A novel catalytic system with a Ru trisbipyridine unit covalently tethered to a B12 derivative has been used for photochemically driven catalytic reactions using triethanolamine as the reductant. A catalytic system using DODOH complexes can lead to reduction products or alkene regeneration depending upon the reaction conditions. These catalytic B12 and model complex systems all utilize a... 

After exploring intermolecular reactions, White and coworkers utilized complex Ll/Pd11 to catalyze the intramolecular oxidative cyclization of 4 to synthesize the macrolide 5 with moderate yield and good regioselectivity (Scheme 7) [23]. Further studies on substrate scope demonstrated that this catalytic system was compatible with various carboxylic acids as nucleophiles, such as aryl acids, vinylic and alkyl acids, leading to the generation of 14- to 19-membered macrolides with remarkable levels of selectivity. 

Since systems 3.4 are hydrolyzed faster, by up to an order of magnitude, than similar derivatives of salicylic acid, EMs of up to 10 may be estimated. However, it is not generally possible to measure EMs systematically because the necessary control - the corresponding intermolecular reaction - is often too slow to be observed above background. The most relevant measure of catalytic efficiency, for comparison with similar reactions in enzyme active sites (where pH is not simply meaningful) is the ratio of the rates of the pH-independent reactions (Eig. 2.1) in the presence and absence of the catalytic group. We use this parameter in the discussion which follows. 

Palladium Catalysts Palladium catalysts are effective and powerful for C—H bond functionalization. Carbene precursors and directing groups are commonly used strategies. Generally, sp3 C—H bond activation is more difficult than sp2 C—H bond activation due to instability of potential alkylpalladium intermediates. By choosing specific substrates, such as these with allylic C—H bonds, palladium catalytic systems have been successful. Both intramolecular and intermolecular allylic alkylation have been developed (Scheme 11.3) [18]. This methodology has presented another alternative way to achieve the traditional Tsuji-Trost reactions. 

The intermolecular asymmetric Heck reaction, a palladium-catalysed carbon-carbon bond forming process, is an efficient method for the preparation of optically active cyclic compounds.[1] Very recently, a new catalytic system has been developed based on palladium complexes having l-[4-(5)-tert-butyl-2-oxazolin-2-yl]-2-(5)-(diphenylphosphino)ferrocene (1) as the chiral ligand121 (Figure 5.2), which we have shown to be efficient catalysts for the enantioselective intermolecular Heck reaction of 2,3-dihydrofuran (2).[3] In contrast to complexes derived... 

It is possible to remove hydrogen catalytically, with formation of new carbon-carbon bonds, especially by means of aluminum chloride. The main importance of this process lies in the preparation of condensed ring systems in the aromatic series. It is convenient to differentiate an intermolecular reaction such as the formation of perylene from naphthalene from an intramolecular reaction such as formation of the same product from 1,1 -binaphthalene 269... 

Terminal alkynes undergo oxidative coupling in the presence of the GuGl-TMEDA catalytic system in [G4GiIm]PF6 under aerobic conditions to produce 1,3-diynes. " Intermolecular Pauson-Khand reactions of strained alkenes with alkynes and Go2(GO)g were performed in [G4GiIm]PF6 either thermally or in the presence of... 

Barbas and researchers identified that the diamine la TFA salt can catalyse the asymmetric intermolecular direct aldol reactions of a,a-dialkylaldehydes with aromatic aldehydes (Scheme 9.2). The bifunctional catalytic system exhibited excellent reactivity to give products with moderate diastereo- and enantioselectivities. Notably, L-proline is an ineffective catalyst for this class of aldol reactions. The re-face attack of an enamine intermediate on an aryl aldehyde was proposed, causing the observed stereochemistry. 

One important advantage of the intermolecular carbene insertion reactions is that simple starting materials can be employed and accordingly there is no need for the construction of complex substrates in advance. However, the intermolecular process requires a delicate balance between electronic and steric effects for metal carbenoids. On the other hand, there are several obstacles to be overcome, including chemo-, regio-, and enantioselectivity. Fortunately, great efforts have been devoted in the past decade and a series of carbene precursors and chiral Rh catalysts have been developed, so satisfactory yields and ee can be obtained in some catalytic systems. Generally, suitable carbene precursors, such as donor/acceptor diazo compounds, could reduce the chance of side product formation due to carbene dimerization. 

The pioneer work of enantioselective C(sp )—H bond functionalization featuring a Pd /Pd catalytic cycle was first reported by Baudoin, Clot, and cowvorkers. The electronwvithdrawing group in the ortho position of substrate 83 was crucial to obtaining the p-arylation product (Scheme 5.31). Furthermore, aryl chlorides are also able to be the coupling partners although the reactions have to be performed at an elevated temperature. The intermolecular arylation products were synthesized in moderate yields and ee. However, it provides a blueprint for developing more efficient catalytic systems. 

The presence of several different ionic particles and therefore, centres with different reactivity, should contribute to the values of the chain growth and chain termination reaction rate constants. The presence of associated, nonassociated and isomeric forms of catalyst particles, the influence of electrolytic dissociation, intramolecular and intermolecular interaction, leading to the formation of catalytic complexes is the reason for the presence of different centres in ionic catalytic systems. 

A more efficient and more generahy applicable cobalt-catalysed Mizoroki-Heck-type reaction with aliphatic halides was elegantly developed by Oshima and coworkers. A catalytic system comprising C0CI2 (62), l,6-bis(diphenylphosphino)hexane (dpph 73)) and Mc3 SiCH2MgCl (74) allowed for intermolecular subshtution reactions of alkenes with primary, secondary and tertiary alkyl hahdes (Scheme 10.25) [51, 53]. The protocol was subsequently applied to a cobalt-catalysed synthesis of homocinnamyl alcohols starting from epoxides and styrene (2) [54]. 

The first example of the asymmetric intermolecular Mizoroki-Heck reaction was reported by Hayashi and coworkers [8] in 1991. This involved the asymmetric arylation of 2,3-dihydrofuran (1) with aryl triflates using a palladium/(7 )-BINAP (BINAP = 2,2 -bis(diphenylphosphino)-l,F-binaphthyl) catalytic system (Scheme 11.4). 

Shibasaki and coworkers [18] carried out the intermolecular asymmetric Mizoroki-Heck reaction with dihydrodioxepines 20 using the palladium-(50-BlNAP catalytic system (Scheme 11.11). The product 21 was obtained in yields up of to 86% and with up to 75% ee. When the aryl group on the triflate 13 was changed, the enantioselectivity was not found to vary appreciably. 

The oxidative addition of disilanes occurs to palladium complexes of isonitrile ligands and platinum complexes of trialkylphosphine ligands as part of tiie catalytic silylation of alkynes and aryl halides. The addition of stannylboranes to Pd(0) complexes has also been reported,and the addition of diboron compounds to many metal systems, such as Pt(0) complexes (Equation 6.67), is now common. These reactions all occur with metal complexes that do not undergo intermolecular reactions with alkane C-H bonds, let alone C-C bonds. Thus, the Lewis acidic character of these reagents must accelerate the coordination of substrate and cleavage of the E-E bonds. 

In 2008, Diaz-Requejo, Perez, and coworkers developed a new silver-based catalytic system which proceeded the direct intermolecular amination of alkanes [33]. This new catalyzed system employed complexes [Tp Ag]2 8 as catalyst and Phl=NTs ([Ag]/[PhI=NTs]=l/20) as the nitrene source. The reaction was carried out in neat alkane at 80°C for 4 h. Linear and branched alkanes were converted to corresponding isomeric mixtures of amides in moderate to excellent yields (Scheme 12). The amination/amidation was favored at tertiary sites over secondary and primary sp C-H bonds of alkanes, and only a few examples were observed at primary sp C-H bonds. The reaction was inhibited when 2,6-di-tert-butyl-4-methylphenol (BHT) was present. Chloroalkanes were observed when CCI4 was used as solvent. These evidences indicated that the mechanism involved radical species. 

Lateral interactions influence the reactants, products, intermediates and even transition states for a reaction. Reactant molecules likely adsorb in different local environments and are therefore exposed to different lateral interactions depending upon the relative number, type and position of neighboring adsorbates. Stochastic kinetic methods provide the best hope of capturing these molecular differences. Traditional deterministic modeling of catalytic systems average over the smface coverage and thus provide only a mean field description. Individual smface sites, as well as intermolecular interactions, however, can be... 

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